In order to relief a sudden change of engine sound during the transition to recovery control of a particulate removing filter or during returning from the recovery control, a system of treating exhaust gas of an engine includes an exhaust-gas particulate removing filter in an exhaust path, closes down the opening of an inlet throttle valve or an exhaust throttle valve to a recovery opening smaller than a normal opening during recovery of the particulate removing filter, and returns the opening from the recovery opening to the normal opening after the recovery of the particulate removing filter. The opening of the inlet throttle valve or the exhaust throttle valve during the closing process from the normal opening to the recovery opening and during the returning process from the recovery opening to the normal opening is changed at a gradually decreasing or gradually increasing rate of change of the opening.